Clear-air turbulence, wikipedia. Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual cues such as clouds, and is caused when bodies of air moving at widely different speeds meet.[1] The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 metres (23,000–39,000 ft) as it meets the tropopause.
Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus cloud can also indicate high probability of CAT. CAT can be hazardous to the comfort, and even safety, of air travel. Detection[edit] Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with conventional radar,[7] with the result that it is difficult for aircraft pilots to detect and avoid it.
Factors that increase CAT probability[edit] Jet stream[edit] Rossby waves caused by this jet stream shear and the Coriolis force cause it to meander. Vertical[edit] Laminar-turbulent transition, wikipedia. The plume from an ordinary candle transitions from laminar to turbulent flow in this Schlieren photograph. The process of a laminar flow becoming turbulent is known as laminar-turbulent transition. This is an extraordinarily complicated process which at present is not fully understood. However, as the result of many decades of intensive research, certain features have become gradually clear, and it is known that the process proceeds through a series of stages. "Transitional flow" can refer to transition in either direction, that is laminar-turbulent transitional or turbulent-laminar transitional flow. While the process applies to any fluid flow, it is most often used in the context of boundary layers due to their ubiquity in real fluid flows and their importance in many fluid-dynamic processes.
History[edit] Reynolds’ experiment on fluid dynamics in pipes Reynolds’ observations of the nature of the flow in his experiments Reynolds' publications in fluid dynamics began in the early 1870s. Langmuir Turbulence, wikipedia. In fluid dynamics, and oceanography, the term Langmuir Turbulence[1] refers to a turbulent flow with a coherent Langmuir circulation structures that exist and evolve over a range of spatial and temporal scales. These structures arise through an interaction between the ocean surface waves and the currents.
In the upper ocean Langmuir circulations are special case where the turbulent structures exhibit a dominant cell size. In general it is expected that Langmuir turbulence is a global ocean phenomenon[2] and not confined to gentle wind conditions or shallow water ways (as with most observations of Langmuir circulation). An important consequence of the Langmuir turbulence are deeply penetrating jets.[3] These features occur between counter rotating rolls and can inject tubulent kinetic energy to depths well below the depth scale for the surface waves (Stokes drift depth scale). See also[edit] Coriolis–Stokes force Notes[edit] Landau–Hopf theory of turbulence, wikipedia. In physics, the Landau–Hopf theory of turbulence, named for Lev Landau and Eberhard Hopf, was until the mid-1970s the accepted theory of how a fluid flow becomes turbulent.
The theory says that as a fluid flows faster, it develops more and more Fourier modes. At first a few modes dominate, but under stronger forcing the modes become power-law distributed, as in Kolmogorov's theory of turbulence. Landau, L. D. (1944). "К проблеме турбулентности" [On the problem of turbulence]. Doklady Akademii Nauk SSSR 44: 339–342. Hopf, E. (1948). Magnetohydrodynamic turbulence, wikipedia. Magnetohydrodynamics (MHD) deals with what is a quasi-neutral fluid with very high conductivity. The fluid approximation implies that we focus on macro length and time scales which are much larger than the collision length and collision time respectively.
In this article we will discuss MHD turbulence which is observed when the Reynolds number of the magnetofluid is large. Incompressible MHD equations[edit] The incompressible MHD equations are where u, B, p represent the velocity, magnetic, and total pressure (thermal+magnetic) fields, and represent kinematic viscosity and magnetic diffusivity. The total magnetic field can be split into two parts: (mean + fluctuations). The above equations in terms of Elsässer variables ( ) are where .
The important nondimensional parameters for MHD are The magnetic Prandtl number is an important property of the fluid. Is around . The Reynolds number is the ratio of the nonlinear term of the Navier-Stokes equation to the viscous term. Of the flow is quite large. . Microturbulence, wikipedia. Microturbulence is a form of turbulence that varies over small distance scales. (Large-scale turbulence is called macroturbulence.) Stellar[edit] Microturbulence is one of several mechanisms that can cause broadening of the absorption lines in the stellar spectrum.[1] Stellar microturbulence varies with the effective temperature and the surface gravity.[2] Magnetic nuclear fusion[edit] Microturbulence plays a critical role in energy transport during magnetic nuclear fusion experiments, such as the Tokamak.[6] Oceanography[edit] References[edit] Jump up ^ De Jager, C. (1954).
External links[edit] Landstreet, J. Turbulence kinetic energy, wikipedia. In Reynolds-averaged Navier Stokes equations, the turbulence kinetic energy can be calculated based on the closure method, i.e. a turbulence model. Generally, the TKE can be quantified by the mean of the turbulence normal stresses: TKE can be produced by fluid shear, friction or buoyancy, or through external forcing at low-frequency eddie scales(integral scale).
Turbulence kinetic energy is then transferred down the turbulence energy cascade, and is dissipated by viscous forces at the Kolmogorov scale. This process of production, transport and dissipation can be expressed as: where:[1] is the mean-flow material derivative of TKE; is the turbulence transport of TKE; is the production of TKE, and is the TKE dissipation. The full form of the TKE equation is By examining these phenomena, the turbulence kinetic energy budget for a particular flow can be found.[2] Computational fluid dynamics[edit] [edit] where This assumption makes modelling of turbulence quantities (k and Boundary conditions[edit] Turbulence modeling, wikipedia. Turbulence modeling is the construction and use of a model to predict the effects of turbulence.
Averaging is often used to simplify the solution of the governing equations of turbulence, but models are needed to represent scales of the flow that are not resolved.[1] Closure problem[edit] A closure problem arises in the Reynolds-averaged Navier-Stokes (RANS) equation because of the non-linear term from the convective acceleration, known as the Reynolds stress, Closing the RANS equation requires modeling the Reynold's stress Eddy viscosity[edit] Joseph Boussinesq was the first practitioner of this (i.e. modeling the Reynold's stress), introducing the concept of eddy viscosity.
. , the turbulence eddy viscosity, has been introduced. Which can be written in shorthand as where is the mean rate of strain tensor is the turbulence eddy viscosity is the turbulence kinetic energy and is the Kronecker delta. Prandtl's mixing-length concept[edit] where: is the mixing length. Among many others[who?] . Notes[edit] Kolmogorov's Turbulence. Mixing length model.
Vortex. Quantum turbulence, wikipedia. Quantum turbulence is the name given to the turbulent flow – the chaotic motion of a fluid at high flow rates – of quantum fluids, such as superfluids which have been cooled to temperatures close to absolute zero. Introduction[edit] A fake vortex tangle representing quantum turbulence in a cubic volume and showing the quantized vortices The turbulence of classical fluids is an everyday phenomenon, which can be readily observed in the flow of a stream or river. When turning on a water tap, one notices that at first the water flows out in a regular fashion (called laminar flow), but if the tap is turned up to higher flow rates, the flow becomes decorated with irregular bulges, unpredictably splitting into multiple strands as it spatters out in an ever-changing torrent, known as turbulent flow.
Despite having no viscosity, turbulence is possible in a superfluid. This was first suggested theoretically by Richard Feynman in 1955,[1] and was soon found experimentally. The two-fluid model[edit] Wake turbulence, wikipedia. This picture from a NASA study on wingtip vortices qualitatively illustrates the wake turbulence. Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components, the most important of which are wingtip vortices and jetwash.
Jetwash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent, but of short duration. Wingtip vortices, on the other hand, are much more stable and can remain in the air for up to three minutes after the passage of an aircraft. Wingtip vortices occur when a wing is generating lift. Air from below the wing is drawn around the wingtip into the region above the wing by the lower pressure above the wing, causing a vortex to trail from each wingtip. Lift is generated by high pressure below the wing and low pressure above the wing. Wake turbulence is especially hazardous in the region behind an aircraft in the takeoff or landing phases of flight.
Take-off. Wave turbulence, wikipedia. When wave amplitudes are small (which usually means that the wave is far from breaking) only those waves exist that are directly excited by an external source. When, however, wave amplitudes are not very small (for surface waves when the fluid surface is inclined by more than few degrees) waves with different frequencies start to interact. That leads to an excitation of waves with frequencies and wavelengths in wide intervals, not necessarily in resonance with an external source. It can be observed in the experiments with a high amplitude of shaking that initially the waves appear which are in resonance, then both longer and shorter waves appear as a result of wave interaction.
The appearance of shorter waves is referred to as a direct cascade while longer waves are part of an inverse cascade of wave turbulence. Statistical wave turbulence and discrete wave turbulence[edit] The subject of DWT, first introduced in Kartashova (2006), are exact and quasi-resonances. Notes[edit] Category:Turbulence, wikipidia. Ch7. Chap8. Lctr-notes634. INTRODUCTORYLECTURES on TURBULENCEPhysics, Mathematics and ModelingJ. M. McDonough. Turbulence, wikipedia. Flow visualization of a turbulent jet, made by laser-induced fluorescence. The jet exhibits a wide range of length scales, an important characteristic of turbulent flows. Laminar and turbulent water flow over the hull of a submarine In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and flow velocity in space and time.
Flow in which the kinetic energy dies out due to the action of fluid molecular viscosity is called laminar flow. While there is no theorem relating the non-dimensional Reynolds number (Re) to turbulence, flows at Reynolds numbers larger than 5000 are typically (but not necessarily) turbulent, while those at low Reynolds numbers usually remain laminar. Features[edit] Turbulence is characterized by the following features: Irregularity: Turbulent flows are always highly irregular. Examples of turbulence[edit] and pressure where ).